Terahertz spectrometer

ABSTRACT

A terahertz spectrometer includes: a terahertz-wave generating element; an elliptical mirror; an optical lens configured to apply a terahertz wave generated by the terahertz-wave generating element, obliquely to a focusing plane of a first focus of the elliptical mirror; and a terahertz-wave detecting element arranged at a second focus of the elliptical mirror.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application claims priority to Japanese Patent Application JP2008-053802 filed in the Japanese Patent Office on Mar. 4, 2008, the entire contents of which is being incorporated herein by reference.

BACKGROUND

The present application relates to a technique of using electromagnetic waves in a band of about 0.1×10¹² THz to 100×10¹² THz.

A known technique of generating or detecting terahertz waves is terahertz time-domain spectroscopy (THz-TDS). As known in the art, the terahertz time-domain spectroscopy is suitable for use in imaging samples because the THz-TDS utilizes terahertz waves that define ultra-short pulses, as short as about 100 femtoseconds. Therefore, the terahertz time-domain spectroscopy attracts attention in various technical fields such as industry, medical care, biotechnology, agriculture and security.

In the terahertz time-domain spectroscopy, a pulse light beam emitted from an ultra-short laser source is split into a pump beam and a probe beam. The pump beam is focused on a terahertz-wave generating element. In the terahertz-wave generating element, a current flow or electrical polarization lasts for about subpico seconds, generating a terahertz wave having field amplitude proportional to the time derivative. The terahertz wave is focused by an optical system on a terahertz-wave detecting element. At this point, the probe beam is applied to the terahertz-wave detecting element. Then, the terahertz-wave detecting element generates a carrier. The carrier is accelerated by the electrical field of the terahertz wave and changed to an electric signal. The time at which the probe beam reaches the terahertz-wave detecting element is delayed, thereby measuring the time waveform the terahertz wave has in the amplitude electric field. The time waveform is Fourier-transformed, thereby determining the spectrum of the terahertz wave.

In an apparatus that performs the terahertz time-domain spectroscopy, an optical system guides, to the sample, the terahertz wave generated in the terahertz-wave generating element, and guides, to the terahertz-wave detecting element, the terahertz wave generated in the sample. In most cases, as shown in FIG. 1, this optical system has four parabolic mirrors PM1 to PM4.

This optical system is a transmission type that detects the terahertz wave that has passed though the sample. A reflection-type optical system is available, which detects the terahertz wave reflected by the sample (see, for example, Jpn. Pat. Appln. Laid-Open Publication No. 2004-191302). The parabolic mirrors may be replaced by small-aperture optical lenses, rendering the lens system small (see, for example, PCT National Publication No. 2003-518617).

SUMMARY

If the optical system incorporates small-aperture lenses, however, the terahertz wave emanating from the sample will travel off the lens diameter. In this case, the terahertz wave will not be guided to the terahertz-wave detecting element if the angle of incidence of the terahertz wave applied through the lenses deviates even a little. It is not known whether this problem is due to a change in the refractive index of the sample or to the shape of the sample. The value measured of the sample is inevitably poor in reliability.

In consideration of the foregoing, the present application has been made. It is desirable to propose a terahertz spectrometer that can measure samples at higher accuracy than before possible.

In an embodiment, a terahertz spectrometer includes a terahertz-wave generating element, an elliptical mirror, an optical lens, and a terahertz-wave detecting element. The optical lens applies a terahertz wave generated by the terahertz-wave generating element, obliquely to the first focusing plane of the elliptical mirror. The terahertz-wave detecting element is arranged at second focus of the elliptical mirror.

The present application in an embodiment can provide a terahertz spectrometer in which a terahertz wave is applied obliquely to an object, such as a sample, if the object is arranged in the focusing plane having the first focus of the elliptical mirror. The terahertz wave is reflected by the object and guided by the elliptical mirror to the terahertz-wave detecting element. Hence, the terahertz-wave detecting element can detect the terahertz wave reflected by the object even if the transmission path of the terahertz wave reflected by the object changes because the object inclines or because the refection surface of the object has depressions and projections. The terahertz spectrometer according to the present invention can therefore measure a sample at an increased accuracy.

The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.

Additional features and advantages are described herein, and will be apparent from the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram explaining a conventional optical system for guiding a terahertz wave to a sample;

FIG. 2 is a diagram showing the overall configuration of a terahertz spectrometer according to an embodiment;

FIG. 3 is a schematic sectional view showing the configuration of the terahertz-wave transmitting unit of the terahertz spectrometer;

FIG. 4 is a diagram explaining how a beam travels if the reflection surface of the object is parallel to the focusing plane having the first focus;

FIG. 5 is a diagram explaining how the beam travels if the reflection surface of the object inclines to the focusing plane having the first focus;

FIG. 6 is a block diagram illustrating the configuration of a computer; and

FIG. 7 is a schematic top view illustrating the configuration of the terahertz-wave transmitting unit of another embodiment.

DETAILED DESCRIPTION

An embodiment will be described with reference to the accompanying drawings.

(1) Overall Configuration of the Terahertz Spectrometer

FIG. 2 shows the overall configuration of a terahertz spectrometer 10 according to an embodiment of the present invention. The terahertz spectrometer 10 includes an ultra-short laser 11, a dispersion compensation unit 12, a beam splitter 13, a terahertz-wave generating element 14, a time delay element 15, a terahertz-wave detecting element 16, and a computer 17.

The ultra-short laser 11 emits a pulse beam having, for example, a pulse width of about 100 fs, a pulse repetition frequency of about 80 MHz and a central wavelength of about 780 nm. An ultra-short laser source LS is, in practice, a titanium laser or a sapphire laser, emitting femtosecond pulses.

The dispersion compensation unit 12 adjusts the pulse width of the pulse beam coming from the ultra-short laser 11, in a specific direction to achieve inverse correction of the pulse width spread that has resulted from the wavelength-dependency of optical fibers BF1 and BF2.

The beam splitter 13 splits the pulse beam coming from the dispersion compensation unit 12 into an excited pulse beam and a detected pulse beam. The excited pulse beam is guided by a focusing optical unit OU1 to the optical fiber BF1 and thence to the terahertz-wave generating element 14 provided in a terahertz-wave transmission unit TMB. Meanwhile, the detected pulse beam passes through the time delay element 15, is then guided by a focusing optical unit OU2 to the optical fiber BF2 and guided through the optical fiber BF2 to the terahertz-wave detecting element 16 provided in the terahertz-wave transmission unit TMB.

The terahertz-wave generating element 14 generates a terahertz wave that has field amplitude proportional to the time derivative of the excited pulse beam. The terahertz-wave generating element 14 is, for example, a photoconductive antenna or a nonlinear optical crystal such as ZnTe. Note that the photoconductive antenna includes a semiconductor substrate of semi-insulating GaAs or the like, a pair of electrodes formed on the semiconductor substrate and a voltage applying unit for applying a bias voltage between the electrodes.

The time delay element 15 delays the time at which the detected pulse beam reaches the terahertz-wave detecting element 16. In this embodiment, the time delay element 15 has a pair of mirrors 15 a and 15 b. One or both of the mirrors 15 a and 15 b are arranged movable. The optical path that extends between the mirrors 15 a and 15 b can therefore be changed in length to adjust the time at which the detected pulse beam reaches the terahertz-wave detecting element 16.

The terahertz-wave detecting element 16 detects the terahertz wave generated in terahertz-wave generating element 14 and guided through the terahertz-wave transmission path provided in terahertz-wave transmission unit TMB. The terahertz-wave detecting element 16 generates an electric field equivalent to the terahertz wave supplied to the element 16 through the terahertz-wave transmission path provided in the terahertz-wave transmission unit TMB. When the terahertz-wave detecting element 16 receives the detected pulse beam from the time delay element 15, the terahertz-wave detecting element 16 generates a signal that has a time waveform of the electric field intensity equivalent to the terahertz wave. Like the terahertz-wave generating element 14, the terahertz-wave detecting element 16 is, for example, a photoconductive antenna or a nonlinear optical crystal such as ZnTe.

The computer 17 is configured to acquire information from the signal generated by the terahertz-wave detecting element 16. The information represents the condition of the sample arranged in the transmission path provided in the terahertz-wave transmission unit TMB. Further, the computer 17 is configured to appropriately control the time delay element 15 and the terahertz-wave transmission unit TMB.

(2) Configuration of the Terahertz-Wave Transmission Unit

The configuration of the terahertz-wave transmission unit TMB will be described with reference to FIG. 3. The terahertz-wave transmission unit TMB has a base plate 21. On the base plate 21, a movable stage 22 is arranged. The movable stage 22 can move in a horizontal plane (in x-axis direction and y-axis direction) parallel to the base plate 12 and in the vertical direction (z-axis direction) perpendicular to the base plate 12.

That part of the movable stage 22, which does not face the base plate 21 has a passage port 23 for the terahertz wave. A mount plate is provided below the passage port 23. Placed on the mount plate is an object RB (i.e., sample SPL or measurement reference of the sample (control)) to be irradiated with the terahertz wave.

On the back of the movable stage 22, a distance sensor 24 is provided to detect the distance from the object RB. A shield 25 is provided, extending over the front of the movable stage 22 and, of course, the passage port 23. A shielded space is provided between the inner surface of the shield 25 and the front of the movable stage 22. In the shielded space, the terahertz-wave generating element 14, the terahertz-wave detecting element 16, three optical lenses, i.e., aspheric lens 31, elliptical mirror 32 and semispherical lens 33, and a charge coupled device (CCD) 34 are arranged, able to move with respect to the movable state 22, while maintaining a specific positional relationship.

The aspheric lens 31 is made of silicon material, either single crystal or polycrystal. The aspheric lens 31 is provided on that surface of the terahertz-wave generating element 14, from which the terahertz wave emanates. The aspheric lens 31 guides the terahertz wave diverging from the terahertz-wave generating element 14, obliquely to the plane in which the first focus of the elliptical mirror 32 lies.

The object RB may be placed on the mount plate lying below the passage port 23. In this case, the computer 17 adjusts the height of the movable stage 22 so that the surface having the first focus of the elliptical mirror 32 and the reflection surface of the object RB (the surface of the sample SPL or control, or the deep layer of the sample) may lie in the same plane.

The elliptical mirror 32 focuses the terahertz wave reflected from the object RB located on the surface having the first focus, at the detection surface of the terahertz-wave detecting element 16. In this embodiment, the elliptical mirror 32 is arranged to apply the terahertz wave reflected by the object RB, back toward a position where the terahertz wave has been emitted.

More specifically, the components are arranged in two areas AR1 and AR2 demarked by a plane having a normal NL to the focusing plane FF having a first focus F1, as shown in FIG. 4 wherein the component identical to those shown in FIG. 3 are designated by the same reference numbers. That is, the terahertz-wave generating element 14, aspheric lens 31, terahertz-wave detecting element 16 and semispherical lens 33 are arranged in the area AR1, and the elliptical mirror 32 is arranged in the area AR2. More precisely, the elliptical mirror 32 is arranged to guide the terahertz wave, which has been reflected by the object RB, back to the terahertz-wave detecting element 16 provided on the movable stage 22 that lies above the focus plane FF.

The reflection surface of the object RB may be parallel to the focusing plane having the first focus. In this case, as shown in FIG. 4, an incidence angle θ1 and a reflection angle θ2 are equal in view of the law of reflection. The incidence angle θ1 is defined by the axis ILA (axis of incidence) of the terahertz wave irradiated from the aspheric lens 31 and the normal NL passing the first focus F1 in the focusing plane FF (i.e., reflection surface of the object RB). The reflection angle θ2 is defined by the normal NL and the axis RLA (axis of reflection) of the terahertz wave reflected by the focusing plane FF (i.e., reflection surface of the object RB). Thus, the terahertz wave reflected by the reflection surface of the object RB is reflected in the center area of the reflection surface of the elliptical mirror 32 and eventually focused on the detection surface of the terahertz-wave detecting element 16.

On the other hand, the reflection surface of the object RB may incline to the focusing plane FF. In this case, as shown in FIG. 5, the incidence angle θ1 at which the terahertz wave is applied from the aspheric lens 31 and the reflection angle θ2 at which the terahertz wave is reflected by the reflection surface of the object RB are different from each other. Therefore, the terahertz wave reflected by the reflection surface of the object RB is reflected by the peripheral area of the reflection surface of the elliptical mirror 32 and eventually focused on the detection surface of the terahertz-wave detecting element 16.

Since the object RB inclines to the focusing plane having a first focus or the refection surface of the object RB has depressions and projections, the elliptical mirror 32 is designed to focus the terahertz wave reflected by the object RB, on the detection surface of the terahertz-wave detecting element 16, even if the transmission path of the terahertz wave reflected by the object RB changes.

The semispherical lens 33 is made of silicon material, either single crystal or polycrystal. The lens 33 is provided on the detection surface of the terahertz-wave detecting element 16. The semispherical lens 33 suppresses the reflection of the terahertz wave at the terahertz-wave detecting element 16.

(3) Configuration of the Computer

The configuration of the computer 17 will be described. As shown in FIG. 6, the computer 17 has a central processing unit (CPU) 40, a read only memory (ROM) 41, a random access memory (RAM) 42, an operation unit 43, a storage unit 44,a display unit 45, an interface 46, and a bus 47. The bus 47 connects the ROM 41, RAM 42, operation unit 43, storage unit 44, display unit 45 and interface 46 to the CPU 40. The ROM 41 stores programs and preset information items. The RAM 42 is used as work memory for the CPU 40. To the interface 46, at least the time delay element 15 (FIG. 2) and the movable stage 22 (FIG. 3) are connected.

The CPU 40 reads the programs from the ROM 41 and expands them in the RAM 42. In accordance with the programs, the CPU 40 controls, as needed, the storage unit 44, display unit 45 and the interface 46, which perform various processes.

The CPU 40, which has thus expanded the programs in the RAM 42, is considered to have a stage adjusting unit 51, a sample measuring unit 52, an image processing unit 53 and a time-delay adjusting unit 54 as illustrated in FIG. 6.

The stage adjusting unit 51 can move the movable stage 22 in accordance with a distance signal generated by the distance sensor 24 (FIG. 3), setting the movable stage 22 at a prescribed height with respect to the object RB and holding the focusing plane having a first focus of the elliptical mirror 32 and the reflection surface of the object RB in the same plane.

Further, the stage adjusting unit 51 can move the movable stage 22 at a predetermined speed in a predetermined scanning direction, i.e., either the x-axis direction or the y-axis direction, thereby to change the position (focusing position), where the terahertz wave coming from the aspheric lens 31 should be focused on the focusing plane having a first focus of the elliptical mirror 32.

Signals generated by the terahertz-wave detecting element 16 are input to the sample measuring unit 52. More precisely, the sample measuring unit 52 receives two signals (hereinafter called “first detection signal” and “second detection signal”). The first detection signal is input from the terahertz-wave detecting element 16 while the object RB used as a sample SPL, remains arranged on the mount plate lying below the passage port 23. The second detection signal is input from the terahertz-wave detecting element 16 while the object RB used as a control remains arranged on the mount plate lying below the passage port 23. Note that the second detection signal may be stored in the storage unit 44 beforehand and may be acquired from the storage unit 44 whenever necessary.

After receiving the first and second detection signals, the sample measuring unit 52 extracts the amplitude data item and phase data item about the terahertz wave from the first signal, and the amplitude data item and phase data item about the terahertz wave from the second signal. From the difference between the amplitude data items and the difference between the phase data items, the sample measuring unit 52 generates information that represents the condition of the sample SPL.

On receiving the information representing the condition of the sample SPL, the image processing unit 53 causes the display unit 45 to display the condition of the sample SPL, which is represented by the information. The user can therefore visually recognize the condition of the sample SPL.

When the user operates the operation unit 43, instructing the display unit 45 to display the condition of the sample SPL, the image processing unit 53 causes the display unit 45 to display the image represented by the image signal generated by the CCD 34. The user can therefore visually recognize the passage port 23 and the object RB arranged below the passage port 23.

The time-delay adjusting unit 54 is configured to determine a desired length of the optical path that extends between the mirrors 15 a and 15 b of the time delay element 15, from the signal input at the operation unit 43 (not shown), and to move one or both of the mirrors 15 a and 15 b, thereby to set the length of the optical path to the desired value.

(4) Operation and Effect

In the terahertz spectrometer 10 configured as described above, the terahertz-wave detecting element 16 is arranged at the second focus of the elliptical mirror 32, and the terahertz wave generated by the terahertz-wave generating element 14 is applied obliquely to the plane in which the first focus of the elliptical mirror 32 lies.

In the terahertz spectrometer 10, the terahertz-wave detecting element 16 can therefore detect the terahertz wave applied obliquely to the object RB and reflected by the object RB if the object RB is placed in the focusing plane FF (see FIG. 3 and FIG. 4).

Further, in the terahertz spectrometer 10, the terahertz wave reflected by the focusing plane FF having a first focus F1 of the elliptical mirror 32 is guided to the terahertz-wave detecting element 16 arranged at the second focus of the elliptical mirror 32. The terahertz wave reflected by the object RB can be focused on the detection surface of the terahertz-wave detecting element 16 even if the transmission path of the terahertz wave reflected by the object RB changes because the object RB inclines with respect to the focusing plane FF or because the reflection surface of the object RB has depressions and projections (see FIG. 5).

In the terahertz spectrometer 10, the terahertz-wave generating element 14, aspheric lens 31, terahertz-wave detecting element 16 and semispherical lens 33 are arranged in the area AR1, and the elliptical mirror 32 is arranged in the area AR2, the areas AR1 and AR2 being demarked by a plane having a normal NL to the focusing plane FF having the first focus F1 (see FIG. 4 and FIG. 5).

Thus, in the terahertz spectrometer 10, the terahertz wave applied from the area AR1, reflected from the object RB and travelling toward the area AR2 can be applied back toward a position where the terahertz wave has been emitted. The terahertz-wave generating element 14 and the terahertz-wave detecting element 16 can therefore be arranged close to each other. This helps to miniaturize the terahertz spectrometer 10. Moreover, the terahertz-wave generating element 14 and the terahertz-wave detecting element 16 can be closer to each other than in the case where the terahertz wave applied to the object is applied back after passing through the object, even if no optical system is used to change the optical path with respect to the transmission path between the elliptical mirror 32 and the terahertz-wave detecting element 16.

Further, in the terahertz spectrometer 10, the terahertz-wave generating element 14, aspheric lens 31, terahertz-wave detecting element 16 and semispherical lens 33, all provided in the area AR1, are arranged above the focusing plane FF having the first focus F1 (see FIG. 4 and FIG. 5).

This means that the terahertz-wave generating element 14 and the terahertz-wave detecting element 16 can be arranged even closer to one another. The terahertz spectrometer 10 can therefore be made much smaller. For example, the terahertz-wave generating element 14 and the terahertz-wave detecting element 16 may be arrayed to provide a probe. Further, the movable stage 22 can be arranged to avoid the interference of the terahertz wave, more easily than in the case where, in the area AR1, the terahertz-wave generating element 14 and terahertz-wave detecting element 16 are arranged, respectively, above and below the focusing plane FF having the first focus F1.

In the terahertz spectrometer 10, the aspheric lens 31, which is made of silicon material, is used as an optical lens (see FIGS. 3 to 5). The aspheric lens 31 can therefore be adjusted to the terahertz-wave generating element 14 in terms of refractive index. This can reduce the energy loss in the terahertz wave generated by the terahertz-wave generating element 14 and applied to the object RB, which results from the reflection of the terahertz wave at the optical lens. In addition, since a single lens can alone suppress the aberration, the number of lenses used and the sizes thereof are smaller than in the case where a spherical lens or a parabolic mirror is used. This serves to miniaturize the terahertz spectrometer 10, too. No anti-reflection film is bonded to optical lenses for guiding terahertz waves in most cases to reduce the influence of multipath reflection, though anti-reflection film is bonded to the optical lenses for ordinary uses. In view of this, the use of a single lens is particularly useful.

Further, in the terahertz spectrometer 10, the optically system provided between the terahertz-wave generating element 14 (terahertz-wave detecting element 16) and the sample SPL is composed of a single aspheric lens 31 (semispherical lens 33). This more reduces the number of interfaces in which unnecessary reflection may occur than in the case where the optical system is composed of a plurality of lenses. The terahertz wave can therefore be improved in intensity. Ultimately, the measuring accuracy can be enhanced.

In the terahertz spectrometer 10, the computer 17 moves the movable stage 22 in accordance with the distance detected by the distance sensor 24, i.e., the distance from the sample SPL placed at the first focus F 1. The focusing plane FF having the first focus F1 is thereby held in flush with the reflection surface of the sample SPL (see FIG. 3 and FIG. 6).

In the terahertz spectrometer 10, the terahertz wave reflected by the sample SPL (i.e., object used as a sample or a control) can be guided to the terahertz-wave detecting element 16, irrespective of the size or shape of the sample SPL. Thus, various kinds of samples can be measured with high accuracy.

In the terahertz spectrometer 10, the computer 17 moves the movable stage 22, changing the position at which the terahertz wave coming from the aspheric lens 31 is applied, with respect to the focusing plane FF having the first focus F1. The information about the position where the terahertz wave is applied to the sample SPL placed in the focusing plane FF is acquired from the signal generated by the terahertz-wave detecting element 16 (see FIG. 3 and FIG. 6).

The terahertz spectrometer 10 can therefore perform the imaging of a greater part of the sample, enhancing the measuring accuracy. The terahertz spectrometer 10 is advantageous over the Fourier spectroscopy in that the terahertz spectrometer 10 achieves a high S/N ratio and can acquire amplitude data and phase data at the same time.

In the configuration described above, the terahertz wave is applied obliquely to the sample SPL arranged in the focusing plane FF having the first focus F1 of the elliptical mirror 32. The sample SPL reflects the terahertz wave, which is detected by the terahertz-wave detecting element 16 provided at the second focus F2 of the elliptical mirror 32. The terahertz wave reflected by the sample SPL can therefore be focused on the detection surface of the terahertz-wave detecting element 16 even if the transmission path of the terahertz wave reflected by the sample SPL changes because the sample SPL inclines with respect to the focusing plane FF of the first focus or because the refection surface of the sample SPL has depressions and projections. Thus, the terahertz spectrometer 10 can measure the sample at an increased accuracy.

(5) Other Embodiments

The embodiment described above has an aspheric lens 31. This invention is not limit to this, nevertheless. For example, a refractive lens composed of N lenses arranged in a line or such parabolic mirrors as shown in FIG. 1 may be used. That is, one or more optical lenses of any type can be used if they can apply the terahertz wave generated by the terahertz-wave generating element 14, obliquely to the focusing plane FF having the first focus F1 of the elliptical mirror 32. As specified above, the aspheric lens 31 is made of silicon material. Instead, the aspheric lens 31 may be made of any other material as long as the aspheric lens 31 can be adjusted to the terahertz-wave generating element 14 in terms of refractive index.

In the embodiment described above, one terahertz-wave generating element 14 and one aspheric lens 31 are arranged in the terahertz-wave transmission unit TMB. This invention is not limited to this configuration. Two or more terahertz-wave generating elements 14 and two or more aspheric lenses 31 may be arranged in the terahertz-wave transmission unit TMB.

FIG. 7 shows a terahertz-wave transmission unit having two terahertz-wave generating elements and two aspheric lenses. In FIG. 7, the components identical to those shown in FIG. 3 are designated by the same reference numbers. FIG. 7 shows the components as viewed from the shield 25 (FIG. 3) toward the movable stage 22.

In this terahertz-wave transmission unit TMB, two excited pulse beams are applied to two terahertz-wave generating elements 14A and 14B through optical fibers BF1 a and BF1 b, respectively. The optical fibers BF1 a and BF1 b are independent fibers or fibers branched from one fiber. The terahertz-wave transmission unit TMB focuses the terahertz waves applied from the terahertz-wave generating elements 14A and 14B through the aspheric lenses 31A and 31B, respectively, to a sample SPL. The sample SPL reflects the terahertz waves. The terahertz waves thus reflected are reflected by an elliptical mirror 32. A semispherical lens 33 focuses the terahertz waves on a terahertz-wave detecting element 16.

One terahertz-wave detecting element is provided for all terahertz-wave generating elements in the terahertz-wave transmission unit TMB of FIG. 7. The terahertz-wave transmission unit TMB can therefore be smaller than in the case where terahertz-wave detecting elements are provided for the terahertz-wave generating elements, respectively.

In the terahertz-wave transmission unit TMB of FIG. 7, the optical lenses (i.e., elliptical mirror 32 and semispherical lens 33) are used to guide both terahertz waves from the sample SPL to the terahertz-wave detecting element 16. The unit TMB can therefore be much smaller than in the case two elliptical mirrores and two semispherical lenses are used to guide the two terahertz waves, respectively.

Further, as shown in FIG. 7, the incidence angles at which the two terahertz waves are applied to the focusing plane FF having the first focus F1 of the elliptical mirror 32 are equal or similar. Hence, if the aspheric lenses 31A and 31B have different numerical apertures, the spot plane each terahertz wave has with respect to the sample SPL can be altered merely by using a switching unit for switching the terahertz-wave generating element, from the element 14A to the element 14B, or vice versa. Therefore, information items representing different resolutions or different parts of the sample can be acquired for the sample SPL. This helps to increase the measuring accuracy. The switching unit may be one that switches the wavelength or pulse width of the excited pulse beam applied to the terahertz-wave generating elements 14A and 14B, or switches the element to which the excited pulse beam is applied, or switches not only the wavelength or pulse width, but also the element.

The aspheric lenses 31A and 31B may have different incidence angles with respect to the first focus F1 of the elliptical mirror 32, though they have the same numerical aperture or similar apertures. In addition, a switching unit may be used to switch the terahertz-wave generating element, from the element 14A to the element 14B, or vice versa. If this is the case, the depth to which the terahertz waves reach in the sample SPL can be changed to acquire accurate information about the layer structure of the sample. This will increase the measuring accuracy.

The aspheric lenses 31A and 31B may have the same incidence angle with respect to the first focus F1 of the elliptical mirror 32, and may have the same numerical aperture or similar numerical apertures. Using these aspheric lenses 31A and 31B, the terahertz waves may be focused at the same position. Then, the sample SPL will be irradiated with a terahertz wave that has an increased intensity (brightness). As a result, the measuring accuracy will be higher, and the components that cooperate to generate terahertz waves can be more durable, than in the case where only one terahertz-wave generating element is used.

Moreover, the various designs specified above can be combined in any possible way.

In the embodiment described above, the movable stage 22 is moved, moving the optical lenses 31 to 33, terahertz-wave generating element 14 and terahertz-wave detecting element 16 without changing the positional relationship of these components, thereby to hold the focusing plane FF having the first focus F1 in flush with the reflection surface of the sample SPL. This invention is not limited to this, nevertheless. Instead, the mount plate holding the sample SPL may be moved to hold the focusing plane FF having the first focus F1 in flush with the reflection surface of the sample SPL.

In the embodiment described above, the movable stage 22 is moved, moving the optical lenses 31 to 33, terahertz-wave generating element 14 and terahertz-wave detecting element 16 without changing the positional relationship of these components, thereby to change the position where the terahertz wave coming from the aspheric lens 31 is applied to the focusing plane FF having the first focus F1. This invention is not limited to this, nevertheless. Instead, the mount plate holding the sample SPL may be moved to change the position where the terahertz wave coming from the aspheric lens 31 is applied to the focusing plane FF having the first focus F1.

The present application can be utilized in various technical fields such as industry, medical care, biotechnology, agriculture, security, data communication and electronics.

It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims. 

1. A terahertz spectrometer comprising: a terahertz-wave generating element; an elliptical mirror; an optical lens configured to apply a terahertz wave generated by the terahertz-wave generating element, obliquely to a focusing plane of a first focus of the elliptical mirror; and a terahertz-wave detecting element arranged at a second focus of the elliptical mirror.
 2. The terahertz spectrometer according to claim 1, wherein the terahertz-wave generating element, the optical lens, and the terahertz-wave detecting element are arranged in a first area, and the elliptical mirror is arranged in a second area, the first and second areas being demarked by a plane having a normal to the focusing plane passing through the first focus.
 3. The terahertz spectrometer according to claim 2, wherein the terahertz-wave generating element, the optical lens, and the terahertz-wave detecting element are arranged above the focusing plane located in the first area.
 4. The terahertz spectrometer according to claim 1, wherein the optical lens is an aspheric lens made of silicon material.
 5. The terahertz spectrometer according to claim 1, wherein the terahertz-wave generating element is provided in plurality, and the optical lens is provided in plurality, each optical lens being an aspheric lens made of silicon material and arranged in association with one terahertz-wave generating element.
 6. The terahertz spectrometer according to claim 5, wherein the aspheric lenses arranged in association with the terahertz-wave generating element have different numerical apertures.
 7. The terahertz spectrometer according to claim 5, wherein the aspheric lenses arranged in association with the terahertz-wave generating element receive terahertz waves at different incidence angles with respect to the focusing plane of the first focus of the elliptical mirror.
 8. The terahertz spectrometer according to claim 6 or 7, further comprising a switching unit for switching the terahertz wave generated by one terahertz-wave generating element to the terahertz wave generated by another terahertz-wave generating element.
 9. The terahertz spectrometer according to claim 1, further comprising: a sensor configured to detect a distance from an object arranged at the first focus; and a holding unit configured to hold the focusing plane and a reflection surface of the object on the same level based on the distance detected by the sensor.
 10. The terahertz spectrometer according to claim 1, further comprising: a changing unit configured to change the position in the focusing plane, where the terahertz wave coming from the optical lens is applied to the focusing plane; and an acquiring unit configured to acquire information about the position of the object arranged in the focusing plane, using a signal detected by the terahertz-wave detecting element. 